Restructures project with feature-based organization

Refactors project structure to support modular, feature-driven development

Introduces comprehensive language localization support
Adds menu management with role-based access control
Implements dynamic sidebar and theme switching capabilities

Enhances project scalability and maintainability

app/Features/Menu/Services/IMenuService.cs

@@ -0,0 +1,14 @@
+using CalendarServer.Features.Menu.Models;
+
+namespace CalendarServer.Features.Menu.Services;
+
+/// <summary>
+/// Service for retrieving menu structure based on user role.
+/// </summary>
+public interface IMenuService
+{
+    /// <summary>
+    /// Get menu groups filtered by user role.
+    /// </summary>
+    List<MenuGroup> GetMenuForRole(UserRole role, string? currentUrl = null);
+}